Field of the Invention
The invention relates to an arrangement for actuating at least one optical element in an optical system.
The arrangement according to the invention can advantageously be used in particular in optical systems having a plurality of optical elements that are adjustable independently of one another, for example for actuating a facet mirror in a microlithographic projection exposure apparatus. However, the invention is not restricted thereto, but can generally also be used in other optical systems (for example optical systems for material processing), and in particular such systems in which a plurality of optical elements are mounted in each case adjustably in a confined structural space.
Prior Art
Microlithography is used for producing microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus, having an illumination device and a projection lens. In this case, the image of a mask (=reticle) illuminated via the illumination device is projected, via the projection lens, onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In a projection exposure apparatus designed for EUV (extreme ultraviolet radiation, i.e. electromagnetic radiation having a wavelength of less than 30 nm, in particular less than 15 nm), because of the lack of availability of light-transmissive materials, mirrors are used as optical components for the imaging process. Furthermore, in particular in the illumination device of a microlithographic projection exposure apparatus designed for operation in the EUV, the use of facet mirrors in the form of field facet mirrors and pupil facet mirrors as beam guiding components is known e.g. from DE 10 2008 009 600 A1. Such facet mirrors are constructed from a multiplicity of individual mirrors which are designed to be tiltable via flexures in each case for the purpose of adjustment or else for realizing specific illumination angle distributions. In this case, there is also a need to realize a tilting about two (in particular mutually perpendicular) tilting axes.
One problem that occurs here in practice is that for actuating for instance the individual mirrors of a facet mirror (or for actuating other optical elements in arrangements in which the elements are comparatively closely packed), firstly only a narrowly delimited structural space is available and secondly often—for instance during the operation of the projection exposure apparatus—thermal loads of the actuator also have to be minimized, which results in demanding challenges in respect of the actuator design. This holds true all the more since the flexures used generally have unavoidable bearing stiffnesses which have to be overcome during the actuation of the individual mirrors or optical elements.